1. Field of the Invention
The present invention relates generally to cellular communication systems. More particularly, the present invention relates to a device and method in a CDMA (code division multiple access) communication system for performing channel acquisition, set maintenance, location positioning, multipath detection, delay estimation, channel status estimation, and finger assignment after detecting a specific channel on a forward link.
2. Description of the Related Art
FIG. 1 illustrates the various power levels associated with signals on a forward link transmitted by a base station in a conventional CDMA communication system (i.e., IS-95). The forward link is comprised of a pilot channel, a sync channel, a paging channel, a control channel and a traffic channel. The traffic channel is a dedicated channel, whereas the synch channel, the paging channel, and the control channel are common channels. Traffic signals are sent on traffic channels at different transmission power levels, and a sync signal and a pilot signal are sent on the synch channel and the pilot channel at constant transmission power levels. A terminal in this system acquires initial synchronization from the pilot channel received at the constant power level, and then performs finger assignment and set maintenance for handoff by measuring the reception level of the pilot channel. During the set management, the terminal can manage signal information generated from a plurality of base stations by continuously monitoring pilot signals received from a candidate set and a neighbor set as well as from an active set in communication with the terminal. Here, the terminal measures the reception level and delay or relative delay of multipath signal components received from the base stations. If the signal level of a pilot signal received from a base station in the active set drops to T_Drop or below during a call or the signal level of a pilot signal received from a base station in the neighbor set rises to T_Add or higher during a call, the terminal sends a pilot level measurement message to the base station. Upon reception of the message, the addressed base station considers that a handoff occurs and sends a handoff message to the terminal. Through a series of procedures, the terminal implements a handoff as it is travelling.
Windows are set for base stations and the terminal searches a corresponding window for each base station. As the bandwidth of a CDMA communication system becomes wider as in IMT (International Mobile telecommunication)-2000, the time resolution in a receiver becomes correspondingly narrower. Thus, the chip size of a window which the terminal should monitor becomes larger. Further, energy received from one path may be smaller than in an existing narrow-band system by the amount of increased time resolution. Further, if the rate of a pilot channel relative to all other transmission signals is to be reduced due to the wide band, a significant constraint is imposed on the mobile search. Therefore, as a consequence of the implementation of a wideband system, the power of the pilot channel cannot be reduced to or below a predetermined rate.
IMT-2000 supports a high data rate service as compared to the conventional mobile communication system. To accommodate rapid data transmission, a signal should be sent at a higher power level than a low data rate service like voice. Since transmission of a signal at a high power level may adversely affect the entire system capacity in a CDMA communication system, it is necessary to limit available services according to terminal location and channel status. This is performed through the conventional set management. However, the conventional set management has limitations because it is based on a low data rate service. To overcome the limitations associated with conventional set management, a way should be explored in which a terminal can acquire signals from more base stations and estimate the channel status faster and more accurately.
The FCC (Federal Communications Commission) of the United States provides that a terminal should be equipped with a device for informing a user's location within a radius of 125 m for 67% or more time in an emergency. If a terminal can acquire signals from a plurality of base stations during a set management, the signals can help detect the mobile's location. As signals are acquired from more base stations in a neighbor set, the location can be detected more accurately.
However, it is impossible for a terminal nearer to a base station to acquire a signal from another base station because a signal from the former is far stronger than that the latter. Even a terminal located within a handoff region has much difficulty acquiring signals from a plurality of base stations because the power of a pilot channel sent from each base station on a forward link is limited. FIG. 2 illustrates, by example, power measurements of pilot signals sent from a plurality of base stations. Even a terminal near a handoff region cannot distinguish a pilot signal from noise component because of insufficient transmission power of the pilot signal. In this case, a searcher in the terminal detects the pilot signal only if it despreads an input signal for a long time. To accurately and rapidly acquire the pilot signal, the terminal should be provided with a searcher with complex hardware.
In view of the foregoing, it is not easy to estimate the location of a terminal on a forward link. To overcome this problem, an IS-95 system estimates a terminal's location using a power-up function (PUF). To allow the location of a terminal in an emergency to be estimated, the terminal sends a signal on a reverse link at a high power level until a plurality of base stations receive the signal. FIG. 3 illustrates the mobile initiated signal. Upon receipt of a PUF command from a base station, the terminal raises its transmission power until a plurality of base stations can acquire its signal. The base station acquires the signal from the terminal and measures a round trip delay and signal level. Based on the measured information, the distance between the terminal and a corresponding base station can be estimated.
Upon receipt of a command requesting implementation of a PUF from the base station, the terminal sends the PUF using a preamble of a reverse traffic channel as shown in FIG. 3. The base station sets PUF performing positions, intervals between PUF pulses PUF-PERIOD, and other related parameters, and the terminal sends the PUF at the determined locations at a power level INC_PWR for the first pulse and then at a power level a specified amount PWR_STEP higher than the previous power level for a next pulse. The maximum number of pulses that the terminal can send is determined by a parameter TOTAL_PUF. The period of A single PUF is an integer multiple of 16 PCGs and divided into three segments the segments PUF_SETUP_SIZE and INC_PUF_SIZE are sent at a usual power level but the segment COMPLETE_FRAME is sent at a higher power level than usual.
To enable multiple base stations to receive a mobile signal, there are cases where the power of a reverse link should be increased from that for a call by 30-40 dB or higher. This may have a deadly influence on the performance and capacity of the reverse link. In addition, a mobile PUF is limited by a maximum mobile transmission power. The PUF scheme has limitations in its effectiveness of estimating a mobile location if a terminal is positioned where the distance between the terminal and base stations is large or the terminal runs out of battery life.